1. Field of the Invention
The present invention describes a technique to determine when a gaseous bio-decontamination process has been successful.
2. Present State of the Art
The problem addressed by this invention is a method of establishing when gaseous bio-decontamination of a chamber has been successfully completed. The most commonly used technique to show that a bio-decontamination has been achieved is with Biological Indicators (BIs) (see Disinfection, Sterilization, and Preservation, 5th Edition. Block, Lippincott Williams & Wilkins. p 22, and Principles and Practice of Disinfection, Preservation and Sterilization, 3rd Edition. Russell, Hugo and Ayliffe. Blackwell Science. p 708). These are small coupons, usually about 10 mm in diameter, manufactured from stainless steel and inoculated with about one million bacterial spores. Endospores are chosen for this purpose because it is generally accepted that they are one of the more resistant organisms. BIs are placed around the chamber to be decontaminated and then removed at the end of the gassing period and either placed into a growth media and then incubated to see if any of the spores are still viable, or placed into buffer solution and then the number of viable spores are estimated. The use of BIs is time consuming and when they are placed into growth media the results are not generally considered to be definitive for at least seven days. The process of estimating the number of viable spores after placing the BIs in buffer solution is labour intensive and again the results will not be immediately available.
As a result of the time taken to establish if a gaseous bio-decontamination has been successful (because of the delay caused by the analysis methods) it is common practice to ensure gross overkill by using excessive amounts of gas and exposing the chamber for periods which are longer than necessary. In the event of a failure to achieve a kill of the spores on the BI, the process would have to be repeated, thus doubling the time for the bio-decontamination process. Long exposure to excessive amounts of gas increases the time taken to remove the gas from the chamber at the end of the process thus further increasing the overall cycle time.
A method of establishing the point in the gassing process when the micro-organisms have been killed would be of benefit because it would shorten the cycle time and also remove the uncertainty that the process has been successful.
If a population of micro-organisms are subjected to a consistent stress level sufficient to cause kill then it is generally accepted that the time taken to reduce the viable population by a factor of 10 will always be the same. Thus if the initial population is 1,000,000 and this reduces to 100,000 in 2 minutes then in a further 2 minutes the viable population will fall to 10,000. The time taken to achieve a 10 fold reduction, sometimes referred to as a 1 log reduction is called the ‘D’ value (see Disinfection, Sterilization, and Preservation, 5th Edition. Block, Lippincott Williams & Wilkins. p 82-83). The death kinetics of micro-organisms are not always strictly linear, but the ‘D’ value concept is broadly accepted in the field of microbiology.
Hydrogen peroxide vapour has become the decontaminant of choice for gaseous bio-decontamination in the Pharmaceutical Industry (see Lysford J. ISPE Barrier Conference, May 2004, Washington) because the process is rapid, reliable and leaves no residues. It is also environmentally friendly because the vapour can be converted to water and oxygen at the end of the process. It has been established that once the correct stress level has been achieved the ‘D’ value for Geoacillus stearothermophilus endospores is less than 2 minutes (see Resistance of common environmental spores of the genus Bacillus to vapour hydrogen peroxide. Kokubo M. et al. PDA J. of Phar. Sci. & Tech. Vol. 52, No. 5. September/October 1998 p 228-231). Thus, if the test population is 1,000,000 organisms then bio-decontamination would be achieved in 12 minutes once the correct stress level is established. For the purposes of this discussion we will consider how to find the point in a gaseous hydrogen peroxide decontamination cycle when the required stress level has been achieved, but identical arguments apply to other gaseous bio-decontamination processes and other micro-organisms.